I propose to investigate various aspects of the functioning and organization of a gene in Drosophila melanogaster that controls both sex determination and the overall rate of X chromosome gene transcription. This master regualatory gene called Sex-lethal (Sxl; 1-19.2) makes a product that is essential for diplo-X flies but is lethal for haplo-X individuals. The Sxl gene is regulated in response to the X chromosome/ autosome balance through the action of a maternally-synthesized product, made or controlled by the daughterless gene (da; 2-41.5). This cytoplasmic factor appears to operate early in development at a cis-acting regulatory region of the Sxl locus. We will explore the mechanism by which the Sxl gene is activated in order to better understand the nature of self-sustaining epigenetic changes that are involved in the determination of cell fates. Two aspects of the Sxl/da interaction are particularly important for this work. First, conditions exist under which Sxl mutant alleles act as dominant lethals for one sex, but are essential for the development of the opposite sex. Second, effects on Sxl gene activation can be distinguished from effects on Sxl product functioning. We will use these two features to determine the organization of the Sxl locus and to relate that organization to the gene's functions. I have devised schemes which facilitate the isolation of new alleles at Sxl and functionally related genes, and which allow the construction of gene rearrangements. These genetic tools will be particularly useful in developmental and biochemical studies. We will explore the relationship between X chromosome dosage compensation and sex determination, and the relationship between sex determination in soma vs. the germ line. We will attempt to identify the Sxl gene product and follow its regulation. Finally, we will further characterize the da maternal cytoplasmic factor to discover when and how it functions to regulate the Sxl gene.